Sandwich structure with a high load-bearing capacity, as well as methods for the manufacture thereof

ABSTRACT

A sandwich structure based on a composite panel made of a thermoplastic core layer connected to at least one fiber-reinforced thermoplastic layer, in particular in-situ manufactured sandwich panels, comprises a thermoplastic core, which is arranged between two cover layers, in which the core is reinforced with fiber-reinforced thermoplastic reinforcing ribs which extend between the cover layers. Preferred methods for the manufacture are also described.

This application is the National Stage of International Application no.PCT/NL2007/000121 filed May 7, 2007, which claims the benefit ofNetherlands patent application number 1031768 filed May 8, 2006, thecontents of which are incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a sandwich structure, and to methods for themanufacture thereof.

BACKGROUND OF THE INVENTION

Inter alia from EP-A-636463 by the Applicant, sandwich panels which havebeen manufactured in situ are known, which comprise a thermoplastic foamcore layer which is connected to at least one—optionallyfiber-reinforced—thermoplastic cover layer, as well as a method for themanufacture thereof. The term in-situ manufacturing is understood tomean forming the foam core layer in one step and connecting it to atleast one cover layer in a press, starting from an assembly comprising alayer of a thermoplastic which is impregnated with a suitable physicalblowing agent and comprises the at least one thermoplastic cover layer.This assembly is placed in a press and, once the material-dependentfoaming temperature has been reached, the distance between the pressplatens is increased. Following cooling and drying, an in-situmanufactured sandwich panel is obtained. Because of their favorablestrength and weight properties, such sandwich panels are particularlysuitable for use in the transport sector, such as in aviation and spacetravel.

Nowadays, there are also thermoplastic sandwich panels which comprise atleast one fiber-reinforced thermoplastic cover layer and a core layermade of a thermoplastic honeycomb structure. This honeycomb structurecomprises essentially parallel open tubular elements made from athermoplastic which are at right angles to the main surface of thefiber-reinforced thermoplastic cover layer.

With some intended uses of (thermoplastic) sandwich panels, the expectedload is quite considerable and thus there is a need for constructionelements of this type which have improved strength properties.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a sandwich structure basedon an assembly of at least one fiber-reinforced thermoplastic coverlayer and a thermoplastic core layer, in particular of in-situmanufactured sandwich panels having improved strength properties, aswell as methods for the manufacture thereof, or a usable alternativetherefor.

To this end, the invention provides a sandwich structure based on acomposite panel made of a thermoplastic core layer connected to at leastone fiber-reinforced thermoplastic layer, in particular an in-situmanufactured sandwich panel, which structure comprises a thermoplasticcore which is disposed between two cover layers, in which the core isreinforced with fiber-reinforced thermoplastic reinforcing ribs whichextend between the cover layers. Due to the presence of thefiber-reinforced thermoplastic reinforcing ribs, which extend from theone cover layer in the direction of the other cover layer—in other wordsdo not run parallel with the cover layers—in the core, the sandwichstructure according to the invention has a higher (compression- orshear-) strength and bending stiffness than an in-situ manufacturedsandwich panel, as a result of which the sandwich structure can besubjected to higher loads. The core thus consists of several parts,which are separated from one another by the reinforcing ribs. The corecomprises a non-solid thermoplastic structure, e.g. a thermoplastic foamor a thermoplastic honeycomb. A thermosetting or thermoplastic adhesive,which may optionally be reinforced with fibers, may be used in order toconnect the components of the thermoplastic sandwich structure. Ifpossible, the thermoplastic which is present in the core, reinforcingribs and cover layers will itself act as bonding agent, e.g. byhot-welding.

In the context of the present description, the term “composite panel” isunderstood to mean an object having a thickness which is small incomparison to its length and width, and which is composed of athermoplastic core layer and at least one fiber-reinforced thermoplasticcover layer.

Furthermore, the expression “sandwich panel” in the present descriptionrefers to a particular embodiment of a composite panel, that is anobject having a thickness which is small in comparison with the lengthand width of the object, and which is composed of a core which isdisposed between two cover layers. The second cover layer does notnecessarily have to comprise a fiber-reinforced thermoplastic, but mayalso comprise an optionally fiber-reinforced cover layer of anothermaterial, such as a metal plate or a (fiber-reinforced) thermosettingmaterial layer.

A “thermoplastic sandwich panel” refers to a sandwich panel of thistype, which is composed of a thermoplastic core layer and twofiber-reinforced thermoplastic cover layers.

An “in-situ manufactured sandwich panel” refers to a sandwich panel ofthis type, which has been produced by means of the above-describedin-situ foaming technique, in other words a thermoplastic foam layerwhich is disposed between two fiber-reinforced thermoplastic coverlayers.

Within the context of this application, “sandwich structure” refers toan object which also has a thickness which is small in comparison to itslength and width, and which is composed of a thermoplastic core disposedbetween two cover layers, with fiber-reinforced thermoplasticfiber-reinforcing ribs likewise being provided between the cover layers,and being connected thereto.

Incidentally, it should be noted that DE19715529 discloses varioussandwich structures for construction components for airplanes, whichcomprise a foam core and (fiber-reinforced) cover layers. In certainembodiments, the foam core is composed of segments, with each segmentcomprising foam-filled fiber bodies. These segments may be arranged indifferent configurations adjacent to one another, and thus form a row ofsegments. One or more rows of segments stacked on top of one another arearranged between two cover layers and connected thereto. This documentalso discloses a method for manufacturing such sandwich structures, inwhich the connection between the segments themselves and between thesegments and the cover layers is produced—if desired via an additionalfiber-reinforcing layer—by injecting an impregnating agent (resin) intoa cover layer of the sandwich structure in such a manner that the agentalso penetrates between adjacent segments in a row of segments, andsubsequently allowing the impregnating agent to set. These knownsandwich structures are said to have a compressive, shearing and bendingstrength and rigidity adapted to the loads.

Furthermore, DE19515930 discloses a similar sandwich structure in whichfoam-filled extruded sleeves are arranged next to one another andconnected to one another to form a core, which can be accommodatedbetween cover layers in order to form the sandwich.

Other similar sandwich structures which comprise a (foam)core betweencover layers, with reinforced connecting ribs which extend between thecover layers in the core, are known, inter alia, from U.S. Pat. No.5,547,737.

The specific problems associated with thermoplastic sandwich panels,sandwich structures and the manufacture thereof is not mentioned inthese publications.

Advantageously, with the sandwich structure according to the invention,the reinforcing ribs are at right angles to the main surfaces (coverlayers) thereof. Other directions of the reinforcing ribs, insofar asthese extend between the cover layers, are possible and are partlydetermined by the load which is expected to occur during the intendeduse.

In one preferred embodiment of a sandwich structure according to theinvention, the core comprises segments which are arranged adjacent toone another, a segment advantageously being manufactured from athermoplastic sandwich panel, preferably an in-situ manufacturedsandwich panel which comprises a thermoplastic core body, preferably afoam body, which is covered on at least one surface with afiber-reinforced thermoplastic layer. Such segments which generally havea relatively great length in comparison to the width and/or thicknessmay be obtained from an in-situ manufactured sandwich panel by simpledeformation steps, as will be explained in more detail below.

The shape of such a segment is not limited. Examples—viewed in crosssection—vary from round, oval, polygonal, triangular, parallelogram,rectangular to square, and combinations thereof. In view of theabove-described preferred direction of the reinforcing ribs, segmentswith rectangular or square cross sections are preferred.

Such segments may in themselves also be used as lightweight boards,reinforcing beams, door posts, jambs for windows and doors, etc. Thus,the invention also relates to a construction element comprising anelongate thermoplastic foam body which is covered, at least on thelongitudinal sides, with a fiber-reinforced thermoplastic layer.Advantageously, the construction element has a rectangular crosssection. For coupling purposes, the construction element preferably hasa tapered end.

In a further preferred embodiment, a segment comprises a foam body witha rectangular cross section, with at least four sides of the foam bodybeing covered by a fiber-reinforced thermoplastic layer. Thermoplastic(sub)layers and any added reinforcing layers thereof may in this caseoverlap one another or abut one another.

If the foam is an anisotropic foam with elongate cells, as is the casewith an in-situ manufactured sandwich panel, the segments areadvantageously arranged in such a manner that the length direction ofthe foam cells extends in the thickness direction of the structure, inother words is at right angles to a main surface of the structure. Inthis advantageous embodiment, the orientation of the cells supports thepresence of the reinforcing ribs with respect to the compressivestrength. Similarly, this is also true for a thermoplastic honeycombstructure in the core.

According to another preferred embodiment, the sandwich structureaccording to the invention comprises an assembly of at least twosubpanels, each subpanel comprising a (preferably thermoplastic, morepreferably fiber-reinforced thermoplastic) cover layer, which isprovided on at least one side with thermoplastic core parts which arearranged at a distance from one another and which, on at least two sidesand preferably on all sides, are covered with a fiber-reinforcedthermoplastic cover layer, the subpanels being connected to one anotherin such a manner that the core parts of a subpanel are situated in aposition between the core parts of another subpanel.

Sandwich panels are often formed as flat panels, preferably inaccordance with standard dimensions customary in the industry. Incontrast with the above-described embodiments based on separatesegments, this preferred embodiment of the invention leaves thepanel-like nature of the starting materials in the form of panelsessentially intact. The preferably complementary subpanels which canadvantageously be produced by local deformation under the effect of heatand pressure in a simple manner from a sandwich panel, preferably athermoplastic sandwich panel, in particular an in-situ manufacturedsandwich panel, can be connected to one another, for example by means ofa thermosetting adhesive, following positioning. In this embodiment, thereinforcing ribs may not only extend essentially parallel to one anotherbetween the cover layers, but in several directions, preferably at rightangles to one another, which results in a further improvement in thestrength and/or stiffness properties compared to a traditional panel,comprising at least one thermoplastic cover layer and a thermoplasticfoam core.

Depending on, inter alia, the distance between the core parts, and theirshape and dimensions, an additional fiber-reinforced thermoplastic layer(which is also referred to as reinforcing layer below) is advantageouslyprovided, at least between neighboring core parts of a subpanel. Acontinuous reinforcing layer which extends over several core parts canalso be used. As in this embodiment the reinforcing ribs are usuallyformed from a cover layer of the starting material, it may be necessaryto add at least one additional layer when manufacturing a subpanel, inorder to achieve the same overall thickness of the fiber-reinforcedcover layers (except for the local reinforcing ribs) across the entiresurface area. The dimensions of the reinforcing layer, the number ofreinforcing layers and the nature of the materials thereof, includingthe fiber reinforcement, can be tailored to the specific requirements ofthe intended use.

In the above embodiments, the top and bottom cover layers of thesandwich structure are uninterrupted.

Another sandwich structure according to the invention is completely madeup of elongate segments, which segments comprise a thermoplastic corebody having a fiber-reinforced thermoplastic cover layer on itslongitudinal sides, the top side and the bottom side being provided withconnecting flanges extending parallel to the top side and bottom sideand made from a fiber-reinforced thermoplastic cover layer. In thiscase, the top and bottom cover layers of the sandwich structure arecomposed of cover layer parts of the individual segments, which coverlayer parts are connected to one another by means of the connectingedges.

The sandwich structure according to the invention can be manufactured ina variety of ways. The invention also looks upon preferred methods forthe manufacture of a preferably thermoplastic sandwich structureaccording to the invention, comprising a thermoplastic core which isarranged between two (preferably thermoplastic, more preferablyfiber-reinforced thermoplastic) cover layers, in which the core isreinforced with fiber-reinforced thermoplastic reinforcing ribs whichextend between the cover layers, said method comprising the followingsteps:

-   -   a) providing a composite panel made of a thermoplastic core        layer which is connected to a fiber-reinforced thermoplastic        layer;    -   b) folding at least one edge of the composite panel over a        length which is at least equal to the thickness of the panel, so        that a segment is obtained which is covered on at least two        adjacent surfaces with a fiber-reinforced thermoplastic layer;    -   c) repeating steps a) and b) until the desired number of        segments is obtained;    -   d) arranging a number of segments next, preferably adjacent, to        one another between two (preferably thermoplastic, more        preferably fiber-reinforced thermoplastic) cover layers, in such        a manner that each segment touches at least one of the two cover        layers, preferably also an adjacent segment; and    -   e) connecting the cover layers to the segments.

The methods according to the invention including the alternativepreferred method to be discussed below make it possible to obtain athermoplastic sandwich structure with improved strength properties, ashas been described above. Since the core layer of the composite panel ismade from a thermoplastic material, it can easily be compacted to form acompact mass of very small dimensions by local heating to the meltingpoint ±20° C. and applying pressure, while the thermoplastic cover layercan easily be deformed by applying pressure and by being exposed to asufficiently high temperature (near the glass transition temperature). Ahot deformation ram is a suitable tool which can be used to this end. Ifdesired, the core part which is to be compacted in another way may alsobe removed beforehand. As has been mentioned above, additional(fiber-reinforced) thermoplastic layers may be added as reinforcinglayers, preferably in such a manner that these also extend in thethickness direction in the finished sandwich structure. Preferably, stepa) comprises the in-situ formation of a thermoplastic sandwich panelcomprising two fiber-reinforced thermoplastic cover layers with athermoplastic foam layer inbetween, which panel is optionally dividedinto (rectangular) sections of suitable dimensions. Preferably, such apanel or section thereof is folded along two parallel edges, in such amanner that a segment which is covered on at least four sides isobtained. Advantageously, step b) comprises folding the two edges fromopposite sides of a panel or section. In order to obtain particularlystrong reinforcing ribs, in a further preferred embodiment, step b)comprises folding the edges from the same side of a panel or sectionfollowed by folding the edges from the opposite side, so that the edgesfolded from different sides overlap to some degree. As a result of thismeasure, the covered sides of a segment which have been formed byfolding, have a double covering of fiber-reinforced thermoplasticmaterial, which will later serve as reinforcing ribs. If desired,additional reinforcing layers may also be added on one side, on bothsides or on all sides in step b). In step d), the segments can bearranged as a row of segments adjoining one another or at a certaindistance from one another. If segments adjoin one another, the segmentsare also connected to one another in step e). When the segments arearranged at a distance from one another, the intermediate open spacescan be used as flow passages for a fluid, for example as an air-coolingpassage. Such flow passages may also be formed by holes in one or morecorners or recesses in the segments. The segments, which may optionallyadjoin one another, may be stacked on top of one another in severalrows.

In an alternative method for manufacturing a sandwich structurecomprising a thermoplastic core which is arranged between two coverlayers, in which the core is reinforced with fiber-reinforcedthermoplastic reinforcing ribs which extend between the cover layers,subpanels are manufactured first. To this end, the method comprises thefollowing steps:

-   -   f) providing a sandwich panel, preferably a thermoplastic        sandwich panel, more preferably an in-situ manufactured        thermoplastic sandwich panel, comprising a thermoplastic core        layer which is arranged between two cover layers, at least one        of which is a fiber-reinforced thermoplastic cover layer;    -   g) providing one or more interruptions in the fiber-reinforced        thermoplastic cover layer of the panel;    -   h) folding edges of the cover layer which are situated along an        interruption, so that a subpanel is obtained comprising a cover        layer, which is provided with core parts which are arranged at a        distance from one another, which are covered on at least two        sides with a fiber-reinforced thermoplastic cover layer;    -   i) repeating steps f)-h) at least once until the desired number        of subpanels has been manufactured;    -   j) positioning at least two subpanels in such a way with respect        to one another that the core parts of a subpanel are at a        position between the core parts of another subpanel; and    -   k) connecting the subpanels to one another.

The term “interruption” in this context refers to an interruption in thefiber structure of the fiber-reinforced thermoplastic layer. Aninterruption of this kind can be produced by means of any kind of tool,such as a knife, laser, miller or saw. The interruption may optionally,in addition to the interruption of the fiber structure, be accompaniedby the removal of a part, for example a strip of the thermoplasticfiber-reinforced cover layer and/or thermoplastic core. If theunderlying core is not removed, then this will be compacted locallyduring step h) as a result of the pressure and heat exerted duringfolding or bending.

In a preferred embodiment, the interruptions, also referred to asincisions, are provided parallel to one another and at a distance fromone another in step g), resulting in a sandwich structure withreinforcing ribs which run parallel to one another.

In yet a further preferred embodiment, the interruptions in thesubpanels are produced in complementary patterns in step g), so that, inthe finished sandwich structure, the core parts of a subpanel adjoin andare also connected to the foam core parts of another subpanel via thereinforcing ribs.

Advantageously, in step h), the edges of the thermoplastic sandwichpanel which are situated along an interruption are folded over a lengthwhich is at least equal to the thickness of the panel. The resultthereof is that the folded part of the fiber-reinforced cover layer canalso be connected to the other cover layer during step k), thusresulting in a strong construction of the sandwich structure.

As the dimensions of the surface of the original fiber-reinforcedthermoplastic cover layer in which the interruptions are made, are ofteninsufficient to cover the lateral faces of the core parts formed byfolding as well (in other words to form the reinforcing ribscompletely), it is advantageously possible to arrange additionalfiber-reinforced thermoplastic layer parts (reinforcing layers) overand/or near the interruptions. Again, the dimensions of the reinforcinglayer, the number of reinforcing layers and the nature of the startingmaterials thereof, including the fiber reinforcement, can be tailored tothe specific requirements of the intended use. For example, thedimensions of the reinforcing layer can be selected to be such that thereinforcing layer not only covers the bottom and sides of the recessprovided between the core parts, but also extends over non-deformedparts of the fiber-reinforced thermoplastic cover layer next to therecess.

Since the surfaces of the subpanels to be connected are not readilyaccessible, a thermosetting adhesive is preferably used in this step k).The subpanels can also be used individually, e.g. as corrugated sheetsfor roofs, walls for reservoirs, etc. Furthermore, the recesses orspaces in the subpanels may be filled with one or more segments,following which a further cover layer may also be applied on top.Depending on the shape of the recess and the desired degree of filling,the latter may also be effected by means of subsegments, such as smallrectangular blocks, cylinders, etc.

One method for manufacturing a thermoplastic sandwich panel with athermoplastic foam core by means of in-situ foaming is known, forexample, from EP-A1-0636463, as has already been described above.

Such a known in-situ foamed sandwich panel consists of a foamed corelayer, which is covered with two fiber-reinforced thermoplastic coverlayers. A cover layer comprises one or more (fiber-reinforced)thermoplastic materials. The in-situ foaming process consists of severalsteps. The first step is an assembly step, during which a core web,which comprises at least one film made of thermoplastic material, whichmaterial contains a certain amount of a suitable physical blowing agent,is positioned between, for example, two (fiber-reinforced) cover layers,which are usually made of the same thermoplastic material as the coreweb. Subsequently, the assembly of core web and cover layers is placedbetween two press platens in a press. In this position, a foaming stepis performed, in which heat and pressure are added to the assembly firstvia the press platens, so that a connection between the core web and thecover layer or cover layers is produced. Then, during a foaming step,when the temperature has reached a sufficiently high level, the press isslowly opened, as a result of which the distance between the two pressplatens increases. This allows the blowing agent (swelling agent,solvent, chemical blowing agent and/or physically inert gas) to expand,as a result of which the material of the core web starts to foam. Thisexpansion is usually carried out under controlled conditions. In thisway, the core web is foamed, and the connection between the core web andthe cover layer or layers is produced in one manufacturing step withouta separate or additional adhesive being required. Once a predeterminedthickness of the foamed core web has been achieved, the assembly isallowed to cool off during a cooling step. The product obtained in thisway comprises the foamed core web which is covered by two thermoplasticcover layers and connected thereto. In addition, a drying step isusually carried out.

Examples of swelling agents for a core layer comprising thermoplasticfoam, include, amongst others, acetone, methyl ethyl ketone, methylacetate, methyl propionate, nitro ethane, cyclohexane, ether, ethanol,methanol and pentane, as well as mixtures, such as ethanol/acetone andmethanol/methyl acetate. Acetone is a preferred swelling agent.

Examples of suitable thermoplastics for both the core layer and thematrix for the fiber-reinforced cover layers comprise polyetherimide(PEI), polyethersulfone (PES), polysulfone, polyvinylsulfone (PPSU),polyketone, such as polyetheretherketone (PEEK), liquid crystalpolymers, polycarbonate (PC), propylene (PP), polyethylene (PE),polyvinyl chloride (PVC), etc., as well as combinations thereof.Polyetherimide is a preferred thermoplastic. The latter is available indifferent grades from General Electric under the brand name UItem. Glassfibers are preferred as reinforcement. Other inorganic fibers, such asmetal fibers, carbon fibers and organic fibers, such as aramide fiberscan be used in a similar manner, provided they can be deformed to asufficient degree in order to form the reinforcing ribs. It is alsopossible to use natural fibers, in addition to the abovementionedsynthetic fibers. The fibers can be used in the form of mats, fabricsand the like. Directional fibers, in particular unidirectional fiberswhere the fiber direction is matched to the intended use, can also beused advantageously. The core layer, e.g. thermoplastic honeycomb orpreferably thermoplastic foam may optionally be reinforced with fibersof the abovementioned types or with nano particles. Preferably, thethermoplastic material of the core layer is the same as thethermoplastic material in the cover layers. However, it is also possibleto use combinations of different thermoplastics. Suitable examplesthereof include, inter alia, PEI foam covered with cover layers made ofPPSU, PS, PEEK or PC, PES or PPSU foam covered with PPSU or PC coverlayers, and PP foam covered with polyamide, such as nylon cover layers.

The abovementioned examples are similarly true for the additionalreinforcing layers. These may, for example, be used as consolidatedlayer or as prepreg, which may consist of several layers (laminates)which are stacked on top of one another.

If desired, the core may also consist of several layers, which areseparated by an intermediate layer, such as an optionallyfiber-reinforced (thermoplastic) layer, ceramic layer or metal layer.

Other methods for manufacturing an assembly comprising a thermoplasticcore and at least one fiber-reinforced thermoplastic cover layerinclude, inter alia, extruding a thermoplastic foam or thermoplastichoneycomb onto a fiber-reinforced thermoplastic cover layer, preferablybetween two such cover layers, and gluing a thermoplastic core layer toa fiber-reinforced thermoplastic cover layer. In general, the adhesiveused should be able to withstand the temperatures required fordeformation.

If desired, an outer fiber-reinforced layer which has not beenimpregnated with plastic may be applied to a segment or sandwichstructure, which outer fiber-reinforced layer is then injected withresin (vacuum injection), so that a double bond is achieved, i.e.adhesion between the injected resin and the thermoplastic, on the onehand, and permeation of the dry fiber-reinforced layer, which ispartially connected to the thermoplastic, on the other hand.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below with reference tothe attached drawing, in which:

FIGS. 1 (a)-(c) diagrammatically show a method for manufacturing athermoplastic foam segment which is covered on four sides with afiber-reinforced thermoplastic cover layer;

FIGS. 2( a)-(c) diagrammatically show an alternative method formanufacturing such a segment;

FIG. 3 diagrammatically shows a sandwich structure made from thesegments according to FIG. 1 or 2;

FIGS. 4( a)-(c) diagrammatically show an alternative method formanufacturing a segment covered on all sides with cover layers;

FIGS. 5( a)-(e) diagrammatically show another embodiment of a method formanufacturing a sandwich structure from subpanels according to theinvention;

FIG. 6 diagrammatically and in cross section shows another embodiment ofa subpanel which may be used with the method according to the invention;

FIG. 7 shows yet another embodiment of a method for manufacturing asegment in a diagrammatical way;

FIGS. 8( a)-(c) show yet another embodiment of a method formanufacturing a subpanel; and

FIG. 9 shows a sandwich structure according to the invention composed ofsegments which have been coupled together.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 diagrammatically shows an example of an embodiment of a methodfor manufacturing a segment which comprises a foam core which is coveredon four sides. Such a segment may be used separately, but may also beused in the method for manufacturing a sandwich structure according tothe invention.

FIG. 1( a) shows a cross section of a thermoplastic sandwich panel 10,which comprises an in-situ foamed foam core layer 12, which is coveredon the top and bottom sides with fiber-reinforced thermoplastic coverlayers 14 and 16, respectively. In a first step, the right-hand side, asillustrated in FIG. 1( a), is deformed from above using a hot moldingstamp 18, as a result of which the respective edge section of the topcover layer 14 is folded over up to the bottom cover layer 16, with theheat compacting the respective section of the foam core layer 12. Theboundary between the deformed section and the non-deformed section isreferred to below as “folding line” and is illustrated in the figure asa dashed line. In a subsequent step, the left-hand edge of the bottomcover layer 16 is now folded over from the bottom (see FIG. 1( b)),resulting in an elongate segment 20 with rectangular cross section,which comprises the foam core layer 12, which is covered on four sideswith a fiber-reinforced thermoplastic cover layer 14 and 16,respectively. See FIG. 1( c).

FIGS. 2( a)-(c) show an alternative method, starting from athermoplastic sandwich panel 10, in which the lateral faces of thesegment are covered with the fiber-reinforced thermoplastic cover layerby first folding over the top cover layer 14 at two opposite edges usinga molding stamp 18 in the manner described with reference to FIG. 1, andsubsequently the bottom cover layer 16. Thus, a segment 20 is created,which again comprises a foam core layer 12, the top and bottom 22 and24, respectively, of which are covered with a fiber-reinforcedthermoplastic cover layer 14 and 16, respectively, and the lateral faces26 with a double fiber-reinforced thermoplastic cover layer 14 and 16.If desired, additional fiber-reinforced thermoplastic reinforcing layers70 may be applied during or after folding. Depending on the thickness ofthe panel, it may be desirable to remove the core layer up to or up toand including the folding line or at least a part thereof, beforebending or folding the cover layer edges.

FIG. 3 shows how a series of segments 20 are arranged with the lateralfaces 26 adjacent to one another between two fiber-reinforcedthermoplastic cover layers 30. The entirety is connected with the aid ofan adhesive, a connection being achieved both between adjacentthermoplastic cover layers 14, 16 of adjacent segments 20 and betweenthe cover layers 14, 16 of the segments 20 and the cover layers 30. Thesandwich structure 40 obtained in this manner comprises thermoplasticfoam core parts 42 originating from the foam core 12 of the segments 20,which are separated from one another by reinforcing ribs 44 made offiber-reinforced thermoplastic originating from the original coverlayers 14 and 16, and which foam core parts 42 are accommodated on thetop and bottom side between the cover layers 30.

In the explanation given above, rectangular segments are used.Obviously, other shapes are also possible, such as triangles whose tipalternately points upwards or downwards, resulting in a sandwichstructure, with the reinforcing ribs forming saw teeth, as it were.

FIGS. 4( a)-(c) diagrammatically illustrate a method, which can be usedfor manufacturing segments covered on all sides and, depending on thelength of the segment, for manufacturing cube-shaped blocks. To thisend, the four corners are removed from a rectangular sandwich panel 10or a section thereof, as illustrated in FIG. 4( a), following which therespective edges are successively folded over along the folding linesindicated by dashed lines. Thus, a (block-shaped) segment 20 is created,which is covered on all sides 22 (top), 24 (bottom), 26 (side) with afiber-reinforced thermoplastic cover layer 14 or 16. Block-shapedsegments can, for example, be used as local reinforcing elements.

The next FIGS. 5( a)-(e) show the steps of an alternative method formanufacturing a sandwich structure according to the invention. Asandwich panel 10 comprises a foam core layer 12 between twofiber-reinforced thermoplastic cover layers 14 and 16, respectively. Thetop cover layer 14 is cut (see FIG. 5( a)), so that parallel lines ofcut 50 (illustrated by a dotted line) are created as interruptions ofthe fiber structure in the length direction of the sandwich panel. In asubsequent step, the edges of the incisions are folded over, for exampleusing a hot stamp, as is illustrated in FIGS. 1 and 2, followed byconsolidation using a consolidation stamp, in such a manner that thefolded sections of the cover layer 14 extend up to the other coverlayer. See the subpanel 60 in FIG. 5( b), which comprises a flat coverlayer 16, to which strip-shaped foam core parts 42 are applied on oneside, and the other sides of which (except the head end), i.e. the topside 22 and lateral faces 26, are covered with parts of the top coverlayer 14. Another sandwich panel is cut in a similar manner and deformedto form a subpanel 60′, as defined above. See FIG. 5( c). The twosubpanels 60 are then positioned in such a manner with respect to oneanother that the foam core parts 42 of one subpanel 60 engage inrecesses between the foam core parts 42 of the other panel 60′. Ingeneral, the subpanels 60, 60′ are connected to one another with the aidof an adhesive (and/or of heat and pressure), inter alia via thedeformed thermoplastic cover layers 14, 16. Thus, a sandwich structure40 is created with cover layers 30 which are made locally (in this casealternately on the top and the bottom side) of double cover layers 14and 16 of the subpanels 60, 60′, as well as double reinforcing ribs 44which extend from the top cover layer 30 to the bottom cover layer 30.In this embodiment, the reinforcing ribs 44 are also made from theoriginal cover layers 14 and 16. It will be understood that, inter aliadepending on the thickness of a sandwich panel 10 and the distancebetween the cutting lines 50, it may be necessary, before forming thefoam core parts, to provide one or more additional fiber-reinforcedthermoplastic layers over or near the cutting lines, which are deformedand folded at the same time during the subsequent folding operation ordeformation. See also FIG. 8. By modifying the shape of the recesses andthus of the core parts, it is also possible to produce a curved subpanelor a curved sandwich structure.

It will be understood that the recesses can also be filled withindividual segments, such as for example those according to FIGS. 1, 2and 7. If necessary, an additional thermoplastic reinforcing layer maybe provided on top, which is optionally provided with fiberreinforcement.

FIG. 6 shows a subpanel 60 partially in cross section, in which the topcover layer 14 is cut in both the length and the width direction, with anumber of edges already having been folded over along cutting lines 50.In the intermediate panel formed in this way, it is for example possibleto accommodate block-shaped segments 20 according to FIG. 4, and coveredwith a (fiber-reinforced) cover layer.

FIG. 7 shows yet another alternative method for forming segments. Inthis case, a thermoplastic foam strip 12 is placed between twofiber-reinforced thermoplastic cover layers 14, 16 which extend in atleast two directions beyond the foam strip 12. This starting materialcan also be obtained by partially removing the (foam)core of a sandwichpanel or part thereof from the sides. In subsequent steps, theprotruding parts of the cover layers 14, 16 are folded along the lateralfaces of the foam strip and, depending on the length of the protrudingparts, folded again, if desired. Such segments 20 can be used formanufacturing a sandwich structure as illustrated in FIG. 3. A segmentcan also be manufactured from a core, which is covered with athermoplastic cover layer, preferably on four sides.

As illustrated in FIGS. 8( a)-(c), it is possible, if desired, to lay anadditional fiber-reinforced thermoplastic layer 70 over a cutting line50, following which the underlying foam structure is compacted with ahot deformation stamp and the cover layers are folded over to form edgesof the foam core parts. See subpanel 60 in FIG. 8( c).

FIG. 9 shows two segments which are coupled together and consist of athermoplastic core 12, which is provided on the longitudinal sides, i.e.the top side and the bottom side, with a fiber-reinforced thermoplasticcover layer 14 and 16, respectively, each of which also covers alongitudinal side, and also at least one protruding edge of the othercover layer and form a coupling part 100 in that position. Theprotruding coupling parts 100 are parallel to the respective bottom andtop side, which are partially also tapered in order to compensate forthe thickness of a coupling part. Such a segment can, for example, bemanufactured in the way illustrated in FIGS. 1, 2 and 7, with themodification that the folded parts have a length which is greater thanthe thickness of the starting sandwich panel.

What is claimed is:
 1. A method for manufacturing a sandwich structurecomprising a thermoplastic core, which is arranged between two coverlayers, in which the thermoplastic core is reinforced withfiber-reinforced thermoplastic reinforcing ribs, which extend betweenthe cover layers, which method comprises the following steps: a)providing a composite panel, made of a thermoplastic core layer, whichis connected to a fiber-reinforced layer, the fiber reinforced layercomprising a thermoplastic material and having at least one edge; b)folding at least one edge of at least the fiber-reinforced layer of thecomposite panel over a length which is at least equal to the thicknessof the composite panel, so that a segment of the composite panel isobtained which is covered on at least two adjacent surfaces with thefiber-reinforced layer, the folding being performed using heat andpressure, wherein at least the fiber-reinforced layer is heated to atemperature higher than a glass-transition temperature of thethermoplastic material of the fiber-reinforced layer; c) repeating stepsa) and b) at least once such that at least two segments of the compositepanel are obtained; d) arranging at least two of the segments next toone another between two cover layers in such a manner that each of theat least two segments touches at least one of the two cover layers andsuch that the fiber-reinforced layer of at least one of the segments ispositioned between the at least two segments and forms afiber-reinforced thermoplastic reinforcing rib of the sandwich panel;and e) connecting the cover layers to the at least two segments to formthe sandwich structure.
 2. The method as claimed in claim 1, in whichstep a) comprises the in-situ foaming of a thermoplastic sandwich panel.3. The method as claimed in claim 1, in which the fiber-reinforced layercomprises at least two edges and step b) comprises folding the at leasttwo edges of at least the fiber-reinforced layer over opposite sides ofthe composite panel.
 4. The method as claimed in claim 1, in which atleast two fiber-reinforced layers are provided in step a), eachfiber-reinforced layer having at least two opposed edges and step b)comprises folding at least one edge of each of the at least twofiber-reinforced layers from the same side of the composite panel,followed by folding at least one opposed edge of each of the at leasttwo fiber-reinforced layers from an opposite side of the compositepanel.
 5. The method as claimed in claim 1, in which step b) comprisesfolding all edges of the composite panel, in such a manner that thesegments are covered on all sides with the fiber-reinforced layer.